Circulating or dynamic fluidized bed reactors in which large amounts of solid particles are entrained by the gases flowing through the reactor chamber, are separated from the gases, and are thereafter recycled to the reactor, have long been used for various chemical processes and have recently also been employed as combustors and gasifiers. In heretofore known applications the separation of solids is carried out in a vertical cyclone separator having a hopper-shaped bottom. In these cyclone separators the cylindrical vortex chamber is connected to a gas discharge pipe for guiding the gases upwards whereas the solids are recycled from the vortex chamber to the reactor through a stand pipe via a gas lock. The gas lock prevents the reactor gases from flowing back into the cyclone through the stand pipe. Most commonly, a mechanical gas lock is employed. In more advanced designs a fluidized bed of solid particles in a U-shaped pipe is utilized as a gas lock. The solids recycling system, however, is complex and expensive, particularly in high temperature reactors. In addition, part of the air required for fluidizing the gas lock flows upwards in the stand pipe thereby detrimentally affecting the separation of solids, in particular the separation of light and fine particles. Furthermore, the rising gas flow decreases the transport capacity of the stand pipe.
In the center of a conventional cyclone, a substantial underpressure and a high axial flow velocity are created causing a suction effect on the stand pipe. The suction flow usually has no tangential velocity so that almost all of the solid material of the flow is transported out through the center pipe of cyclone. A recycling system provided with a conventional cyclone is therefore extremely sensitive to the suction flow from the stand pipe and accordingly requires a reliable gas lock.
In steam boiler applications, the use of a conventional cyclone results in a disadvantageous design since a conventional cyclone divides the boiler into a combustion chamber and a separate convection part following the cyclone, the equipment for recycling the solids being disposed therebetween.
In addition, mechanical gas locks have a relatively short life span and, particularly under hot conditions, are frequently subject to malfunction.
It is also known to locate a conventional cyclone including the entire solids recycling system inside the reactor space. This arrangement has, however, severe disadvantages, such as corrosion and erosion of the cyclone, since cooling of the supporting structure is difficult to achieve. Furthermore, as in conventional cyclones, the system is sensitive to the suction flow from the stand pipe.
It is accordingly an object of the present invention to overcome the drawbacks of conventional cyclone separators.